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Need More Top End


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2004 Malibu Response LXI with a 383 Hammerhead Engine

current Prop is a 13 X 13 Stainless

Max RPM with this prop is 5200 at 3900 feet above sea level

My top end is 45 mph max with a skier and i would like to gain another 2 mph or more.

I have tried different prop configurations but th e current prop that I have tried is giving me the best top end.

Any suggestions on to get a little more speed out of my boat? Can the engine be reprogrammed or "chipped"? Any suggestions or info?

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Actually in an '04 you've got a 350ci Hammerhead, the 383 wasn't introduced until '05. But for top end, I would argue that your motor will hold its own against the 383 in that boat.

Elevation is hurting you. You're dialed in (as you suspect) on the prop, since 5200 is where the rev limiter sits. Have you hit that? It doesn't engage on some boats. If that's the case, then it's possible to eek out a little by propping it down just a bit more. As far as chips, they don't exist for these boats. I have heard rumor of outfits in Colorado that know how to flash these motors since they deal with much more extreme altitudes than 4k feet. Figure 3% power loss for every 1k feet of altitude, so you're down about 12%.

Exactly what prop is it that you're running? Have you played with the cup at all?

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Don't take this the wrong way, but unless you put it on a trailer you're not going to get another 2 mph out of it unless you get down to sea level.

That said, I assume you are using a GPS to verify your top end.

I am surprised that you are getting 5200 rpm at 3900 ft asl, particularly with a 13x13, but you may try adding cup or stepping up a touch to a larger prop and dropping the rpm's closer to 4900-5000. (larger prop meaning larger than 13 pitch) If you want to get down to the nity-gritty, a full tank of fuel will help with speed, as will a tonneau cover.

Peter.

edit: My mistake - I see you have a 13x13 stainless, which must be a CVP prop. This will correlate to a 13x12 (or less) CNC'd prop from Acme, so I would suggest trying a 13x12.625.

Edited by SmoothWaterMan
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I don't think you can do it with a prop. Either a supercharger or a turbocharger is your best solution at that altitude. You could run a light boost so as to not cause detonation. You don't need much. I have not looked into it myself but one of my old buddies did some research ten years ago and he found a setup for a chevy 350 marine engine.

He never went through with that project. Instead he sold the boat and bought a hotrod.

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2004 Malibu Response LXI with a 383 Hammerhead Engine

current Prop is a 13 X 13 Stainless

Max RPM with this prop is 5200 at 3900 feet above sea level

My top end is 45 mph max with a skier and i would like to gain another 2 mph or more.

I have tried different prop configurations but th e current prop that I have tried is giving me the best top end.

Any suggestions on to get a little more speed out of my boat? Can the engine be reprogrammed or "chipped"? Any suggestions or info?

No arguments that you need it, but I'm just curious, Why?

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You are fighting, as mentioned, air density. A prop with a bit more pitch could help out if you have a little reserve in the power department, you will need to determine which CVP 13x13 prop you actually have, there should be one of three marks after the 13x13: nothing, M or MD. Each of those steps is about a 1/2 pitch + increment. Your prop does not sound like an M or MD as you are spinning more than 100rpm/mph. I have done the CVP to Acme change and found a little gain in top end. I added a lot of cup for my particular application and it works quite well, but my powerplant is not production.

You can tune on your CVP prop a little for a small gain, sharpen all the leading edges and make the surface as rough as you can to provide the prop with more grip.

On the engine side, you can improve the airflow with a couple of items, try a set of 1.6 ratio rocker arms and/or try a ZZ4 camshaft. The cam provides a significant improvment to the standard GM marine cam but also verify which cam the Hammerhead uses. Also ensure you are at 32-33 degrees BTDC spark advance @ 4500 rpm to optimize combustion, it will be around 10-12 degrees BTDC at idle. You might also try to get the coldest air possible to the engine as it will be more dense than the warmer air under the hood. You can plumb a couple of cold air tubes back to the rear cup holder holes, you can simply cut the bottom of the cup holders away and attach the tubes to that and make an airbox at the inlet side to keep out the warm air. They work very well.

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